The present invention relates to a method and a device for controlling a four wheel drive transmission system for an automotive vehicle, and more particularly relates to a method and a device for controlling such a four wheel drive transmission system, in which the main transmission mechanism of the vehicle is an automatic transmission device.
The present invention has been described in Japanese Patent Application Ser. No. 60-188824 (1985), filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Application and the claim and the drawings thereof; a copy is appended to the present application.
In Japanese Utility Model Laying Open Publication Ser. No. 47-203 (1972), Japanese Patent Laying Open Publication Ser. No. 55-72420 (1980), and SAE (Society of Automotive Engineers) publication serial no. 710615, none of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law, there are disclosed variations upon the theme of fitting a four wheel drive vehicle such as a so called full time four wheel drive vehicle with an automatic transmission system. These publications disclose the concept of feeding engine output rotational power via an automatic transmission device of a per se known sort to the input side of a center differential device, said center differential device having two output sides, a rear wheels output side and a front wheels output side, and providing differential action between its said rear wheels output side and its said front wheels output side: in other words constraining the total sum of the incremental rotational amounts of its rear wheels output side and its front wheels output side (optionally including scale factors) to be substantially equal to twice the incremental rotational amount of its input side while allowing said total incremental rotational amount sum to be substantially arbitrarily distributed between its rear wheels output side and its front wheels output side. The rear wheels output side of said center differential device is connected to the input side of a differential device of a per se known sort for providing differential action between the left rear wheel and the right rear wheel of the vehicle, while similarly the front wheels output side of said center differential device is connected to the input side of a differential device also of a per se known sort for providing differential action between the left front wheel and the right front wheel of the vehicle. And there is disclosed the concept of providing said center differential device with a control clutch, which when engaged substantially prevents said center differential device providing such differential action and instead, more narrowly, constrains each of the incremental rotational amounts of its said rear wheels output side and its said front wheels output side to be individually substantially equal (again, optionally with scale factors) to the incremental rotational amount of its said input side; while, on the other hand, when said center differential device control clutch is released, said center differential device is allowed to provide the aforementioned differential action.
Thus, by selectively either engaging or disengaging said center differential device control clutch, selective differential action can be provided or not provided between the rear wheels and the front wheels of the vehicle, as desired. Typically, in fact, differential action is desired between the rear wheels and the front wheels of the vehicle, when the vehicle is rounding a corner. On the other hand, differential action is in fact not desired between the rear wheels and the front wheels of the vehicle, when the vehicle is being operated in a high power output mode such as when climbing a relatively steep hill or other slope, or alternatively when engine braking is desired, or alternatively when fine control of the vehicle is particularly required. This is because such differential action between the rear wheels and the front wheels of the vehicle deteriorates good vehicle power driving characteristics, tends to reduce the effectiveness of engine braking, and impairs fine control of the vehicle. Therefore, there is a requirement for control of the selective engagement and disengagement of the center differential device control clutch, and it might be conceived of to perform such control clutch control by manual operation by the driver. However, the problem with this is that such control is distracting to the driver and also its effectiveness depends upon the driving skill of the driver. Accordingly, it is not realistic to expect fine manual control of such a central differential device control clutch to be provided by the driver, in view of the diversity of other control responses required from said driver during vehicle operation, especially in the above outlined case that the transmission system of the vehicle is an automatic transmission, since the demands for case of use, imposed by the user public upon such a power train configuration incorporating an automatic transmission, are inevitably more exigent than the demands imposed upon a power train incorporating a manual transmission.